Reflective polarizers having extended red band edge for controlled off axis color

ABSTRACT

Reflective polarizers, which transmit one polarization and reflect the other polarization, have an extended red band edge in the off-axis transmission spectrum to control off axis color (OAC) hue. To control the OAC hue, the red band edge of the off-axis transmission spectrum, or OAC spectrum, is shifted beyond the boundary for red light, or to at least 600 nm. Even more preferably, the OAC spectrum is extended further into the red than any red emission peak of an associated lamp providing light into a system in which the reflective polarizer is used. The concept applies equally well to any type of reflective polarizer, such as multilayer reflective polarizers, cholesteric reflective polarizers, and the like. The resulting reflective polarizers are very useful for many different applications, such as liquid crystal displays.

This application is a continuation of U.S. Ser. No. 08/690,527, now U.S.Pat. No. 5,808,794, filed Jul. 31, 1996.

BACKGROUND

This relates generally to reflective polarizers which transmit onepolarization and reflect the other polarization. More particularly, thisapplication relates to reflective polarizers having an extended red bandedge in the off-axis transmission spectrum to reduce off axis color hue,or to select a particular hue.

One important use for reflective polarizers is in a light-recycling modeto provide brightness increase (gain) in liquid crystal display (LCD)applications. In these applications, the reflective polarizer is used inconjunction with a light-recycling cavity, a light source, and a liquidcrystal panel. Maximum luminance is achieved when the reflectivepolarizer is used in a brightness enhancement mode, such that light ofthe reflected polarization is "recycled" into the transmittedpolarization by the reflective polarizer in combination with the lightrecycling cavity. Examples of such light recycling systems are describedin copending and commonly assigned U.S. patent application Ser. No.08/402,134, which is incorporated herein by reference.

The off axis color (OAC) problem with multilayer-type reflectivepolarizers is described in copending and commonly assigned U.S. patentapplication Ser. No. 08/402,041, which is incorporated herein byreference. OAC is also a problem with other types of reflectivepolarizers, such as cholesteric reflective polarizers. The multilayerreflecting polarizers described in the above mentioned U.S. patentapplication Ser. No. 08/402,041 show a very red or yellow OAC. In otherwords, the OAC has a distinct reddish or yellowish "hue". This isbecause the red band edge of the off axis transmission spectrum isshifted toward the blue. As a result, almost all p-polarized red light(600 nm and greater) is transmitted at off normal incidence, but some ofthe blue and green is reflected, giving the light at off normalincidence a definite reddish appearance. This hue is very objectionablein some applications, such as in the backlit LCD systems, where colorcontrol is highly important.

SUMMARY

To overcome the drawbacks in the art described above, and to providevarious other advantages will become apparent upon reading andunderstanding the present specification, the present invention providesa reflective polarizer with an extended red band edge to reduce off axiscolor (OAC). The reflective polarizer has a OAC red band edge of atleast 600 nm at a 45 degree angle of incidence, and has an averagetransmission from 400-800 nm of less than 20% for light polarizedparallel to the extinction axis at normal incidence. This results in amore color balanced system in which the color bands across the visiblespectrum are more evenly reflected, thus controlling the hue of the OAC.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, where like numerals refer to like elements throughoutthe several views:

FIGS. 1A, 1B and 1C show the transmission spectra of three differentprior art multilayer reflective polarizers;

FIG. 2 shows the transmission spectra of a typical CCFT;

FIG. 3 shows the transmission spectra of a reflective polarizer havingan OAC red band edge at about 670 nm at a 60 degree angle of incidence;

FIG. 4 shows the reflective polarizer of the present invention in aliquid crystal display system.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration a specific embodiment in which the invention may bepracticed. It is to be understood that other embodiments may be utilizedand structural changes made without departing from the spirit and scopeof the present invention.

Reflective polarizers act to transmit light of one polarization andreflect light of another polarization. The light that is transmitted ispolarized parallel to the so-called transmission axis and the light thatis reflected is polarized parallel to the so-called extinction axis.Reflective polarizers can function as a light-recycling film to provideincrease in brightness (gain) when used in conjunction with alight-recycling cavity, a light source, and an absorbing polarizer suchas is used in a liquid crystal display (LCD). This maximum luminance isthe preferred state when the multilayer reflective polarizer is used asa brightness enhancement film.

Off axis color (OAC), or the colorful, iridescent look of a reflectivepolarizer at off-normal incidence, results from the incomplete andirregular transmission of p-polarized light through the transmissionaxis of the polarizer at angles away normal incidence. OAC will changewith angle of incidence, thus, a particular OAC is specific to aparticular angle of incidence. OAC can be seen graphically by measuringthe transmission of p-polarized light polarized parallel to thetransmission axis of the polarizer at off-normal incident angles. Forpurposes of the present description, the spectrum showing thetransmission of light polarized parallel to the transmission axis of thepolarizer at off-normal incidence will be referred to as the OACspectrum. This characteristic of the OAC spectrum is one of the causesof the colorful or iridescent look of a reflective polarizer at anglesaway from the normal.

For multilayer-type reflecting polarizers, OAC is due in part to theout-of-plane (z) index mismatch between adjacent layers, as described inU.S. patent application Ser. No. 08/402,041. That application describesthat to reduce OAC, the multilayer reflective polarizer should have aminimized out-of-plane z-index mismatch to produce a high transmissionspectra for the transmission axis at any angle of incidence forp-polarized light.

The multilayer reflecting polarizers exemplified in the above-mentionedU.S. patent application Ser. No. 08/402,041 showed a very red or yellowOAC. In other words, the OAC of those polarizers had a distinctyellowish or reddish cast or "hue", as it will be referred to herein forpurposes of illustration. The OAC hue is due to the "shift" of the OACspectrum from that of the extinction spectrum at normal incidence whenmoving from normal to higher angles of incidence. For example, thespectra of the multilayer reflecting polarizer shown in FIG. 1A has a670 nm band edge for the extinction axis at normal incidence (curve c),and a 565 nm band edge for the OAC spectrum at a 60 degree angle ofincidence (curve b). The OAC band edge thus "shifts" toward the bluewhen moving from normal incidence to higher angles of incidence. The OAChue for FIG. 1A is very objectionable. This is because the color bandsacross the visible spectrum are not evenly reflected. Almost allp-polarized red light (600 nm and greater) and much of the p-polarizedyellow light (565 nm and greater) for curve b is transmitted, but someof the blue and green is reflected, giving a definite reddish oryellowish appearance to the light transmitted at off normal incidence.

The OAC hue of a reflective polarizer can be a problem in LCD systems,because the OAC red band edge for previously described reflectivepolarizers such as that shown in FIG. 1A is lower than the red emissionpeak for the typical light source used to illuminate the LCD. The lightsource most often used for the application of laptop computers and otherLCD displays is a cold-cathode fluorescent tube (CCFT). Although thespectra for CCFT's made by different manufactures will vary, arepresentative CCFT characteristic emission spectra for an Apple 540cTFT computer is shown in FIG. 2. The location and relative magnitude ofthe red, green, and blue emission peaks of CCFTs are determined byselection of specific phosphors based on desired color temperature. Forthe CCFT of FIG. 2, the red peak is located at about 610 nm, the greenpeak is located at about 550 nm, and several blue peaks are locatedbetween 420 and 500 nm.

Comparing FIGS. 1A and 2, it can be seen that the red emission peak (610nm) for the CCFT of FIG. 2 is higher than the OAC red band edge of thereflective polarizer (565 nm) of FIG. 1A. Since the OAC red band edge islower than the emission peak for the CCFT, most of the red and greenlight emitted by the CCFT is transmitted through the reflectivepolarizer at angles away from the normal. This high transmission for thered and green combined with the lower amount of transmission for theblue results in a yellow appearing OAC. This color is very objectionablein some applications, especially in backlit LCD displays, where displaycolor balance and brightness are extremely important.

EXTENDED OAC RED BAND EDGE

Although it has been recognized that OAC in general is undesirable formany applications, the present invention further recognizes that the hueof the OAC is also an important factor to consider. The presentinvention further recognizes that the hue is affected by the red bandedge of the OAC spectrum at off-normal incidence and by the relativetransmission of red versus green and blue light. In particular, the hueof the transmitted light at off normal incidence is greatly affected bythe position of this red band edge.

In liquid crystal display applications, it is generally preferred thatthe OAC is tuned so that the color shifts from white to blue rather thanwhite to red. Given this preference, certain hues of OAC are morepreferable than others when a reflective polarizer is used in a displaysystem. An off-angle spectrum which is flat at 60 degrees is mostpreferable, as there is no perceived color at angle due to the additionof the reflective polarizer. In the even that a flat OAC spectrum cannotbe achieved, blue OAC is less preferred but still acceptable, whilegreen OAC is still less preferred, and red OAC is least preferred,especially when the reflective polarizer is used in a brightnessenhancing mode in an LCD application.

Most cold cathode fluorescent tubes do not have red emission lines above630 nm. The present invention has recognized that better display colorbalance is possible if the OAC spectrum reflects evenly throughout therange of the lamp spectrum. Or, if this is not possible, the OAC shouldreflect more red light than blue or green light. To accomplish this, thepresent invention has recognized that the OAC spectrum is desirablyextended further into the red than that shown in FIG. 1A. Preferably,the OAC spectrum is shifted beyond the boundary for red light, or atleast 600 nm. Even more preferably, the OAC spectrum is desirablyextended further into the red than any red emission peak of anassociated light source providing light into a system in which thereflective polarizer is being used.

To reduce the hue of any OAC in a reflective polarizer, the presentinvention has identified a useful parameter that will be referred toherein as the "OAC red band edge". For purposes of the presentspecification, the OAC red band edge is defined as the wavelength atwhich the reflectance of the polarizer along the transmission axis atoff normal incidence (the OAC spectrum) for p-polarized light increasesfrom the baseline value to 10 percent of the OAC spectrum's peakreflection value. Although only transmission (T) is shown here,reflection (R) can be assumed to be given by 1- T because absorption ison the order of 1 percent. The baseline reflectance value can be takenin the far red or infrared where no optical layers are tuned to causereflectance. The red band edge is then taken to be the red edge of thelongest wavelength band in the OAC spectrum.

FIG. 3 shows the spectra for a reflective polarizer according to thepresent invention. The OAC red band edge is about 670 nm at a 60 degreeangle of incidence. The average transmission from 400-700 nm for theextinction axis at normal incidence (curve c) is about 1.4%. The Examplebelow describes how the polarizer of FIG. 3 was made. Reflectivepolarizers such as that shown in FIG. 3 are advantageous forapplications such as LCD's, because the hue of any OAC will not appearred or yellow across the useable viewing angle of the LCD. The preferredwavelength at a given angle of the OAC red band edge will vary dependingupon the application with which the reflective polarizer is to be used.The key factors are the wavelengths to be considered and the usefulviewing angle necessary for the particular application. In general,however, to achieve the desirable control of hue of any OAC which may bepresent in any reflective polarizer, the OAC red band edge is desirablyat least 600 nm at a 45 degree angle of incidence, preferably at least620 nm at 60 degrees, more preferably at least 650 nm at 60 degrees, andmost preferably at least 650 nm at a 75 degree angle of incidence. ForLCD applications, where display uniformity is of the utmost importance,the OAC red band edge is desirably at least 630 nm at a 60 degree angleof incidence, is preferably at least 630 nm at an 80 degree angle ofincidence, and is even more preferably at least 650 nm at an 80 degreeangle of incidence.

For multilayer-type reflecting polarizers of the type described in theabove-mention 08/402,041, the shift in the red band edge of the OACspectrum from that of the extinction spectrum was more than expectedfrom the conventional n*d*cos(theta) formula of isotropic materials. Forthe multilayer-type reflective polarizer described in Example 1 belowand shown in FIG. 3, the shift is due to the biaxial birefringence ofthe PEN layer. First consider only the in-plane indices. Curves b and care taken along orthogonal axes. Along the extinction axis, the PENindex is high, approximately 1.83 at 870 nm. However, along thetransmission axis, the in-plane index is much lower, about 1.60. Thismakes the PEN layers much thinner optically in the nonstretch directionthan in the stretch direction. The OAC spectrum is due to a combinationof both the y and z-index differential, and in the limit of normalincidence, is only due to the y-index differential, if there is one. Asmall y-index differential in Example 1 allows one to find the red bandedge of curve b at normal incidence in this limit. The small peaklabeled I on curve "a" is the red band edge for the sample in the limitof normal incidence. This peak is shifted from the red band edge ofcurve c due to the large difference in PEN index (about 0.23) along theextinction and transmission axes for this particular polarizer. The rededge of the peak labeled II on curve "b" is the OAC red band edge at a60 degree angle of incidence. Note how both of these positions areshifted toward the blue from the red band edge of curve "c".

In the multilayer-type reflective polarizers described in the examplebelow and shown in FIG. 3, the low z-index of the PEN layer contributesfurther to the blue shift. This is because the p-polarized light at highangles of incidence senses the low z-index value of 1.49 instead of onlythe in-plane value of 1.62 at 633 nm, resulting in a further reducedeffective index of refraction of the PEN layers. Along the extinctionaxis this latter effect is more pronounced because the difference in thePEN in-plane index to the PEN z-axis index is much greater: 1.83 - 1.49,at 850 nm, resulting in a greater blue shift than that of conventionalisotropic multilayer film stacks, for p-polarized light. For biaxiallyoriented films of multilayer PEN/(low index isotropic polymers), or anybirefringent polymer used to make mirrors, the shift is alsosubstantial. For mirrors made with PEN with equal stretch along the twomajor axes, the in-plane/z-axis index differential is about 1.74 - 1.49.This effect must be accounted for when designing mirrors that mustreflect all visible light at all angles of incidence, i.e., the red bandedge at normal incidence must be placed at a longer wavelength thananticipated from the in-plane indices alone.

For certain reflecting polarizers, such as the multilayer-typereflecting polarizer of FIG. 3, to increase the optical bandwidth of theOAC in a reflective polarizer, the extinction bandwidth of the polarizermust also be increased. In other words, to increase the OAC red bandedge (curve b), the extinction band edge (curve c) must also beincreased. To accomplish this in a multilayer-type reflecting polarizerwith the same values of extinction along the x-axis, more film layerscan be added, tuned specifically to the longer wavelengths. The precisenumber of layers in the film will depend upon the birefringence of thematerials used to make the film. The performance of a PEN/coPENpolarizer with acceptable extinction throughout the visible spectrum,but reddish OAC, is given by the spectra in FIG. 1B. This polarizer ismade with approximately 600 layers, involving a 150 layer feedblock andtwo multipliers. A more detailed description of the polarizer shown inFIG. 1B can be found in U.S. patent application Ser. No. 08/494,416,which is incorporated herein by reference. Curve b shows the tranmissionof p-polarized light polarized parallel to the transmission axis at a 50degree angle of incidence. The average transmission of curve a from400-700 nm is about 86.2%. The average transmission of curve b from400-700 nm is about 84.2%. The average transmission of curve c from400-800 nm is about 11.8%. The OAC red band edge of curve b is about 605nm. The red band edge of curve c is about 745 nm. To increase the OACbandwidth of curve b, and maintain the good extinction of curve c, thenumber of layers was increased to approximately 832. This wasaccomplished with a 209 layer feedblock and two multipliers. Theresulting polarizer performance, with both good extinction and bettercolor balanced OAC, is shown by the spectra of FIG. 3.

When extending the extinction band edge for a reflecting polarizer, theextinction band edge (curve c) in is desirably at least 800 nm,preferably at least 830 nm, and more preferably at least 850 nm, andeven more preferably 900 nm. The precise extinction red band edge willvary, however, depending upon the relationships between the x, y, and zindices of refraction of individual layers of the film and of theirrelationships between film layers.

A different but less desirable method of producing an polarizer havingan extended OAC red band edge without using more layers is simply toredesign the existing layer thickness distribution, i.e. take layersassigned to the midspectrum wavelengths and tune them to longerwavelengths. The spectra of such a film was described in theabove-mentioned U.S. patent application Ser. No. 08/402,041. The spectraof that polarizer is reproduced here in FIG. 1C. This method isunacceptable for many applications, however, because the resulting 600layer polarizer shows significant spectral leaks in both the midspectrumand near IR. These leaks are due to the smaller number of layers tunedto a given wavelength and the difficulty of controlling layerthicknesses so that the reflective polarizer reflects light uniformly atall wavelengths. The average transmission of curve c (extinction atnormal incidence) is about 9% from 400 to 700 nm. However, the averagetransmission from 700 to 800 nm is over 20%. This is significant,because at a 60 degree angle of incidence, that portion of the spectrumfrom 700 to 800 nm blue shifts into the red portion of the visiblespectrum, resulting in a 20% average transmission in the redwavelengths. Such leaks will seriously upset the color balance of abacklit LCD system. In addition, the color balance of curve b is suchthat less red light is reflected than blue or green, and the OAC willhave a red hue at low angles, and a yellow hue at high angles, both ofwhich are unacceptable in a LCD application.

Applying the principles described above, a preferred reflectivepolarizer combines the desired transmission and extinction performancewith the minimum OAC and minimum OAC red band edge. Average transmissionvalues as described herein include both front and back surfacereflections in air. The average transmission from 400-700 nm for thetransmission axis at normal incidence is desirably at least 80%,preferably at least 85%, more preferably at least 90%. The averagetransmission from 400-700 nm for the transmission axis at a sixty degreeangle of incidence is desirably at least 60%, preferably at least 70%,more preferably at least 85%, and even more preferably at least 95%.

The average transmission from 400-800 nm for the extinction axis atnormal incidence is desirably less than 12%, preferably less than 10%,more preferably less than 5%, and even more preferably less than 2%. Theaverage transmission from 400-700 nm for the extinction axis at a sixtydegree angle of incidence is desirably less than 20%, preferably lessthan 15%, more preferably less than 10%, and even more preferably lessthan 5%.

And, as discussed above, the OAC red band edge for a reflectivepolarizer is desirably at least 600 nm at a 45 degree angle ofincidence, preferably at least 620 nm at 60 degrees, more preferably atleast 650 nm at 60 degrees, and most preferably at least 650 nm at a 75degree angle of incidence. For LCD applications, where display colorbalance is of the utmost importance, the OAC red band edge is desirablyat least 630 nm at a 60 degree angle of incidence, is preferably atleast 630 nm at an 80 degree angle of incidence, and is even morepreferably at least 650 nm at an 80 degree angle of incidence. It isalso preferable that the reflectivity of red light is greater than thereflectivity of blue or green light.

The concept of OAC red band edge is not limited to multilayer-typereflective polarizers. The concept applies to any type of reflectivepolarizer. Exemplary multilayer-type reflective polarizers are describedin, for example, the above-mentioned U.S. patent application Ser. No.08/402,041, EP Patent Application 0 488 544 A1, U.S. Pat. No. 3,610,729(Rogers), and U.S. Pat. No. 4,446,305 (Rogers). Other types ofreflective polarizers include cholesteric reflective polarizers or theretroreflecting polarizer described in U.S. Pat. No. 5,422,756 (Weber).Regardless of which type of reflective polarizer is used, it will alwaysbe important to ensure that the red band edge of the OAC spectrumsatisfies the requirements described herein to control the OAC hue. Thereflective polarizers described herein are also useful for a widevariety of applications. The reflective polarizers are useful forbacklit, reflective and transflective LCD applications, window film forpolarization and/or energy control, and many other applications whichwill be apparent to those of skill in the art.

FIG. 4 shows an representative backlit LCD system 100 including areflective polarizer 106 of the present invention. The system is viewedby observer 102. The LCD system 100 also includes an LCD panel 104,light guide 108, lamp 112, and reflector 110. The system could alsoinclude, among other things, brightness enhancement films, prismatic orstructured surface brightness enhancement films, compensation and/orretardation films, diffusers, absorbing polarizers and the like.Although the system shown in FIG. 6 is backlit, the reflective polarizeris also useful for reflective and transflective displays. Otherexemplary LCD systems with which the present reflective polarizers couldbe used are described in copending and commonly assigned U.S. patentapplication Ser. Nos. 08/402,134, 08/514,172, 08/494,776, and08/402,042, all of which are incorporated herein by reference.

EXAMPLE

A coextruded film containing about 833 layers was produced by extrudingweb onto a chilled casting wheel and continuously orienting the film ina tenter. A polyethylene naphthalate (PEN) with an Intrinsic Viscosityof 0.48 dl/g (60 wt. % phenol/40 wt. % dichlorobenzene) was delivered byone extruder at a rate of 75 pounds per hour (34.1 kg/hour) and 70/0/30CoPEN (70 mol % 2,6 NDC and 30 mol % DMI) with an IV of 0.58 dl/g wasdelivered by another extruder at a rate of 85 pounds per hour (38.6kg/hour). These meltstreams were directed to the feedblock to create theCoPEN and PEN optical layers. The feedblock created 209 alternatinglayers of PEN and CoPEN 70/0/30 with the two outside layers of CoPENserving as the protective boundary layers (PBL's) through the feedblock.An approximate linear gradient in layer thickness was produced by thefeedblock for each material with the ratio of thickest to thinnestlayers being about 1.30. After the feedblock a third extruder deliveredthe same 70/0/30 CoPEN as symmetric PBL's (same thickness on both sidesof the optical layer stream) at about 28 pounds per hour (12.7 kg/hour).The material stream passed though an asymmetric two times multiplier(such as that described in U.S. Pat. Nos. 5,094,788 and 5,094,793) witha multiplier ratio of about 1.25. The multiplier ratio is defined as theaverage layer thickness of layers produced in the major conduit dividedby the average layer thickness in the minor conduit. The material streamthen passed though an asymmetric two times multiplier with a multiplierratio of about 1.5. After the multiplier, a thick symmetric PBL wasadded at about 113 pounds per hour (51.4 kg/hour) that was also fed fromthe third extruder. Then the material stream passed through a film dieand onto a water cooled casting wheel using an inlet water temperatureof about 56 degrees F. (13° C.). The optical layers exhibited amonotonically increasing thickness profile from the casting wheel sideto the air side of the film. The thinnest optical layers were closest tothe casting wheel. The CoPEN melt process equipment was maintained atabout 530° F. (277° C.); the PEN melt process equipment was maintainedat about 545° F. (285° C.).; and the feedblock, multipliers, skin-layermodules, and die were maintained at about 540° F. (282° C.).

All stretching was done in the tenter. The film was preheated to about303° F. (150° C.) in about 20 seconds and drawn in the transversedirection to a draw ratio of about 6.7 at a rate of about 25% persecond. The finished film had a final thickness of about 0.005 inches(0.125 mm). The optical spectra are shown in FIG. 3. Curve a is thetransmission at normal incidence of light polarized parallel to thenonstretch direction (i.e., the transmission direction or transmissionaxis). Curve b is the transmission of p-polarized light along this samedirection, but at 60 degrees angle of incidence. Curve c gives thetransmission at normal incidence of light polarized parallel to thestretch direction (i.e., the extinction direction or the extinctionaxis). The red band edge for the extinction axis (curve c), as definedby the wavelength where the transmission value exceeds the 10% value, isapproximately 870 nm. The red band edge for the OAC spectrum (curve b),defined as the wavelength at which the reflectance for the transmissionaxis at off normal incidence increases from the baseline value to 10percent of the band's peak reflection value is approximately 670 nm. Theaverage transmission of curve a from 400-700 nm is 87%. The averagetransmission of curve b from 400-700 nm is 70%. The average transmissionof curve c from 400-800 nm is 1.4%.

In the event that the reflective polarizer is a multilayered-typereflective polarizer, suitable materials which could be used as layersin the film include polyethylene naphthalate (PEN) and isomers thereof(e.g., 2,6-, 1,4-, 1,5-, 2,7-, and 2,3-PEN), polyalkylene terephthalates(e.g., polyethylene terephthalate, polybutylene terephthalate, andpoly-1,4-cyclohexanedimethylene terephthalate), polyimides (e.g.,polyacrylic imides), polyetherimides, atactic polystyrene,polycarbonates, polymethacrylates (e.g., polyisobutyl methacrylate,polypropylmethacrylate, polyethylmethacrylate, andpolymethylmethacrylate), polyacrylates (e.g., polybutylacrylate andpolymethylacrylate), syndiotactic polystyrene (sPS), syndiotacticpoly-alpha-methyl styrene, syndiotactic polydichlorostyrene, copolymersand blends of any of these polystyrenes, cellulose derivatives (e.g.,ethyl cellulose, cellulose acetate, cellulose propionate, celluloseacetate butyrate, and cellulose nitrate), polyalkylene polymers (e.g.,polyethylene, polypropylene, polybutylene, polyisobutylene, andpoly(4-methyl)pentene), fluorinated polymers (e.g., perfluoroalkoxyresins, polytetrafluoroethylene, fluorinated ethylene-propylenecopolymers, polyvinylidene fluoride, and polychlorotrifluoroethylene),chlorinated polymers (e.g., polyvinylidene chloride andpolyvinylchloride), polysulfones, polyethersulfones, polyacrylonitrile,polyamides, silicone resins, epoxy resins, polyvinylacetate,polyether-amides, ionomeric resins, elastomers (e.g., polybutadiene,polyisoprene, and neoprene), and polyurethanes. Also suitable arecopolymers, e.g., copolymers of PEN (e.g., copolymers of 2,6-, 1,4-,1,5-, 2,7-, and/or 2,3-naphthalene dicarboxylic acid, or esters thereof,with (a) terephthalic acid, or esters thereof; (b) isophthalic acid, oresters thereof; (c) phthalic acid, or esters thereof; (d) alkaneglycols; (e) cycloalkane glycols (e.g., cyclohexane dimethanol diol);(f) alkane dicarboxylic acids; and/or (g) cycloalkane dicarboxylic acids(e.g., cyclohexane dicarboxylic acid)), copolymers of polyalkyleneterephthalates (e.g., copolymers of terephthalic acid, or estersthereof, with (a) naphthalene dicarboxylic acid, or esters thereof; (b)isophthalic acid, or esters thereof; (c) phthalic acid, or estersthereof; (d) alkane glycols; (e) cycloalkane glycols (e.g., cyclohexanedimethanol diol); (f) alkane dicarboxylic acids; and/or (g) cycloalkanedicarboxylic acids (e.g., cyclohexane dicarboxylic acid)), styrenecopolymers (e.g., styrene-butadiene copolymers and styrene-acrylonitrilecopolymers), and copolymers of 4,4'-bibenzoic acid and ethylene glycol.In addition, each individual layer may include blends of two or more ofthe above-described polymers or copolymers (e.g., blends of SPS andatactic polystyrene). The coPEN described may also be a blend of pelletswhere at least one component is a polymer based on naphthalenedicarboxylic acid and other components are other polyesters orpolycarbonates, such as a PET, a PEN or a co-PEN.

Particularly preferred combinations of layers in the case of polarizersinclude PEN/co-PEN, polyethylene terephthalate (PET)/co-PEN, PEN/sPS,PET/sPS, PEN/Eastar, and PET/Eastar, where "co-PEN" refers to acopolymer or blend based upon naphthalene dicarboxylic acid (asdescribed above) and Eastar is polycyclohexanedimethylene terephthalatecommercially available from Eastman Chemical Co.

Although specific embodiments have been shown and described herein forpurposes of illustration of exemplary embodiments, it will be understoodby those of ordinary skill that a wide variety of alternate and/orequivalent implementations designed to achieve the same purposes may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. Those of ordinaryskill will readily appreciate that the present invention could beimplemented in a wide variety of embodiments, including different typesof reflective polarizers. This application is intended to cover anyadaptations or variations of the preferred embodiments discussed herein.Therefore, it is intended that this invention be defined by the claimsand the equivalents thereof

We claim:
 1. A liquid crystal display (LCD) system, comprising:a lightsource including a light guide, the light source having a characteristicemission spectra having a red emission peak; an LCD panel; and areflective polarizer disposed in an optical path between the LCD paneland the light source, the reflective polarizer having an off-axis colorred band edge, for light having a 45 degree angle of incidence, that isgreater than a wavelength corresponding to the red emission peak.
 2. Asystem as recited in claim 1, wherein the off-axis color red bandedge isat least 600 nm.
 3. A system as recited in claim 1, wherein the off-axiscolor red bandedge, for light having a 60 degree angle of incidence, isat least 630 nm.
 4. A liquid crystal display (LCD) system, comprising:alight source including a light guide, the light source having acharacteristic emission spectra having a red emission peak at aparticular wavelength; an LCD panel; and a cholesteric reflectivepolarizer disposed in an optical path between the LCD panel and thelight source, the reflective polarizer substantially transmitting lighthaving a first polarization state and reflecting light having a secondpolarization state and having a red bandedge for transmission of lighthaving the first polarization state that changes as an angle of incidentlight on the cholesteric reflective polarizer deviates from normal,wherein the red bandedge does not correspond to the particularwavelength of the red emission peak as the angle of incident lightvaries from normal to 45 degrees.
 5. A system as recited in claim 4,wherein the red bandedge does not correspond to the particularwavelength of the red emission peak as the angle of incident lightvaries from normal to 60 degrees.
 6. A system as recited in claim 4,wherein the system has substantially equal overall transmission of red,green and blue light.